This invention relates to a laser beam printer that is adapted to perform image recording with a laser beam by using electrophotography as a method of forming a latent electrostatic image.
Laser beam printers are capable of recording sharp image at fast speed without making annoying noise which is a major defect of dot printers. Because of these advantages, laser beam printers are gaining increasing acceptance as office machines and in other areas. The operating principle of laser beam printers is to form a latent electrostatic image on a photoreceptor, which is developed in a developing unit to form a toner image that corresponds to the characters or images to be recorded. The toner image is then transferred onto a recording sheet and fused thereto to be ejected from the machine.
Conventional laser beam printers have single scanning densities of their own, mostly 300 dpi (dots per inch) or 400 dpi. The reason why each conventional laser beam printer has only one scanning density is that it is supposed to be used in association with a specific information processor such as a host computer or work station. If a certain information processor is set to handle image information of 300 dpi, the laser beam printer to be connected to that processor need have a scanning density of 300 dpi. On the other hand, if the information processor is set to handle image information of 400 dpi, the laser beam printer to be connected to that processor need have a scanning density of 400 dpi.
FIG. 7 shows how a conventional laser beam printer connected to a host computer operates. In FIG. 7, a laser beam printer 191 comprises a laser scanning unit 192 and a control unit 193 which controls the scanning unit. A host computer 194 is an information processor that supplies text information to the laser beam printer, and the host computer is connected to the control unit 193. In the laser beam printer 191, the scanning unit 192 has a crystal oscillator 196, and the control unit 193 has a crystal oscillator 197. Suppose here that the host computer 194 sends image information of 300 dpi. In this case, the crystal oscillator 196 in the laser scanning unit 192 which operates in synchronism with the rotation of a built-in polygonal mirror (not shown) generates a clock signal that permits a laser beam to scan a photoreceptor (also not shown) at a scanning density of 300 dpi. On the other hand, the crystal oscillator 197 in the control unit 193 will generate a clock signal for processing image information of 300 dpi sent from the host computer 194.
The recent advances in the technology of integrating office machines as accompanied by the development of information processing systems have been remarkable and this has not only led to the introduction of a new information processor such as an "engineering workstation" but also enabled data exchange between a plurality of information processors interconnected on a network. Under these circumstances, there has arisen the possibility that a reading machine that incorporates image information having a scanning density of, say, 400 dpi is used in combination with a text editing machine that handles font information having a different scanning density of, say, 300 dpi. As regards reading machines, standards specified by the C.C.I.T.T. (the Consultative Committee for International Telegraph and Telephone) including the resolution of facsimiles in group G4 have made it common these days for reading machines to incorporate image information of 400 dpi. On the other hand, in the case of text editing systems of the type shown in FIG. 7, printers that perform recording at a scanning density of 300 dpi have now gained wide acceptance, particularly because fonts adapted for 300 dpi are extensively used in word processors and personal computers in both the U.S. and European countries. In this situation, information processors or networks that handle both image and text information may potentially require that laser beam printers perform recording at two scanning densities, say, 300 dpi and 400 dpi.
While the possibility of using different scanning densities has been discussed above with reference to two values, 400 dpi and 300 dpi, this is not the sole case that can be considered. Whichever scanning densities are selected, no single conventional laser beam printers have been capable of dealing with two, three or even more scanning densities if they are to be used in combination. It has therefore been necessary to provide separate printers that are to deal with the respective scanning densities.
However, the use of separate laser beam printers for respective scanning densities not only results in an expensive recording system but also requires an extra space for printer installation.